fix(docs): align architecture diagram connectors

[fang2hou]

Purpose

Correct the connector alignment and direction in the README architecture diagram introduced by #26. This follow-up starts from main after that PR was merged.

Changes and context

• Replace auto-oriented SVG markers and separately sized arrowheads with explicit, consistently sized connector paths. The outlined output previously turned the Static Assets downward arrow into a right-facing arrow.
• Align the horizontal runtime and Discord connectors with their card centers and use a consistent 6px gap between all arrow tips and card borders.
• Show Worker/Static Assets fetch and return with a correctly aligned vertical bidirectional connector. Show OAuth exchange bidirectionally and center the R2 stream label along its rounded route.
• Regenerate the IBM Plex outlined SVG and document why explicit arrowhead geometry is used.

Impact and risks

Documentation/media-only fix; no application behavior, dependencies, authentication, sharing, or deployment configuration changes. The editable source and generated SVG are updated together.

Verification

• mise exec -- node scripts/build-architecture.mjs: 72 label bounds pass.
• Actual browser rendering of the generated SVG: reviewed all eight connectors, including the vertical Static Assets arrowheads, both rounded routes, and the three Discord flow connectors.
• mise run test: quality gate, production build, and the existing test suites pass.